The two guys, Jack and Bill, have decided to try to persuade their wives to
have a bit of partner swapping for the night.
The guys have agreed that if they can pull off the wife swap, when they sit
around the breakfast table the following morning, they will tap their teaspoon
on the side of their coffee mug the number of times that they did it with each
other's wives. Clever enough!
After several drinks that night they succeed! Jack knows it's that time of
the month for his wife and the thought of Bill not knowing this makes him
smile.
The next morning they are all at the breakfast table, slightly hangover and
quite uncomfortable, when Jack proudly taps his teaspoon 3 times against his
coffee mug.
After a brief moment of thinking, Bill takes his teaspoon and taps it once on
the strawberry jam and 3 times on the peanut butter.